The field of the invention is that of telecommunications networks. These networks make it possible to convey all types of information (sound, video, digital data . . . ) from a transmitter to a receiver, each type of information having specific characteristics and giving rise to specific constraints.
More specifically, the present invention concerns a switching element in a telecommunications network of asynchronous time-division type, particularly of ATM type (Asynchronous Transfer Mode), taking into account the characteristics of the communications conveyed so as to privilege certain communications at the expense of others according to these characteristics.
Generally speaking, telecommunications networks can be divided into two large families depending on whether they implement spacial techniques or time-division techniques.
Spacial techniques consist of assigning a permanent physical path to each communication channel and therefore of distributing the communications in space.
Time-division techniques can themselves be subdivided into two subfamilies: synchronous time-division techniques and asynchronous time-division techniques.
In the case of synchronous time-division techniques, the information carriers are time intervals allocated in frames, identified by their position in this frame.
In the case of asynchronous time-division techniques, the carriers of the data transmitted through the network are packets which, in the case of ATM, are referred to as cells and possess a standardized length of 53 octets divided into a 5-octet header and a 48-octet information field, or ATM conduit.
Although the basic format of the cell is the same regardless of the nature of the data conveyed, each of the connections has, depending on its nature, specific characteristics as regards traffic and quality of service (QoS). The characteristics (including the price, the average rate, the peak rate . . . ) of each connection must be specified in a detailed manner in the contract binding the user of the ATM connection and the network operator who provides it to him and are therefore known to the ATM network.
The orientation of a cell within an equipment, a cell in a switch, for example, implies that at least the header of this cell be taken into account by the equipment so as to determine the destination towards which it must be routed by the equipment itself. The various routing mechanisms (spacial switching of cells) are described in the book by Martin de Prycker "ATM--Asynchronous Transfer Mode", published in French by Masson.
Conventionally, a switching equipment comprises a FIFO (First In, First Out) type memory intended to receive the cells, as well as a spacial switching mechanism.
In the course of their transfer through the network, the cells are stored in queues whose filling ratio varies according to the instantaneous load. As a result, the cell transit time may vary from one cell to another for a given communication. This therefore induces a variable delay between two successive cells of the same communication; this delay is called cell jitter.
Now, for so-called synchronous traffic, it is essential to minimize the cell jitter. Non observance of this constraint may result in receiver "famine". This is the case, for example, with telephone communications, video telephony, etc.
A known solution to the cell jitter problem is described in the article "Services audiovisuels et protocoles associes" (Audiovisual services and associated protocols) by Jean-Yves Cochennec and Thierry Houdoin of the CNET, published in L'echo des Recherches, no. 44, 1991.
Indeed, once the maximum deviation of the variation in cell propagation time through the network is known, it is merely necessary to initially defer the forwarding of the information to the receiving terminal using a value equal to this maximum deviation. This maximum deviation is the difference between the longest end-to-end transmission time and the shortest end-to-end transmission time. Existing modeling techniques make it possible to bound these fluctuations in transmission time and therefore the maximum deviation of the cell jitter. Such a mechanism therefore guarantees the absence of receiver famine, even in the most unfavorable cases.
However, even though it solves the cell jitter problem, this mechanism induces a delay which can be detrimental to certain types of traffic, particularly interactive or real-time traffic.
Indeed, certain types of services such as videoconferencing or conventional voice telephony require a minimization of the cell jitter and of the end-to-end transmission time.